1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. In particular, the present invention relates to a semiconductor device including a crystalline thin film transistor and a manufacturing method thereof. Further, the present invention relates to a semiconductor device and an electronic appliance mounted with the thin film transistor.
2. Description of the Related Art
In recent years, development of techniques regarding semiconductor devices including thin film transistors (hereinafter referred to as “TFT”) has been actively pursued. In particular, application to active matrix type display devices and the like using a glass substrate has been advanced.
Semiconductor films used for TFTs can be largely classified into those with crystalline (polycrystalline) structures and those with amorphous structures. TFTs using semiconductor films of the polycrystalline structure has high electron field-effect mobility and high-speed operation is possible. Accordingly, there are advantages that a TFT of a liquid crystal device can be reduced in size and that an aperture ratio is improved. Note that an aperture ratio (of a black matrix layer) is a ratio of an area of an opening portion with respect to an area of one pixel region. Also, in a polycrystalline structure, since a driver IC (integrated circuit) can be formed over the same substrate as a pixel circuit, the number of parts is reduced, which also makes possible size reduction of the an entire liquid crystal display device. Also, in addition to display devices, by forming a TFT over a glass substrate, a semiconductor device can be manufactured at a lower cost than when a silicon substrate is used.
In general, in forming a polycrystalline semiconductor film, first, an amorphous semiconductor film is formed using a semiconductor source gas such as silane, and then crystallizing the amorphous semiconductor film with heat. As the main crystallization method, a laser crystallization method, a thermal crystallization method, a thermal crystallization method using an element that promotes crystallization such as nickel (Ni), or the like is given. In the case of performing crystallization by a thermal crystallization method, an HTPS (high temperature polysilicon) process is used. Since it is possible to make a TFT element fine by using the HTPS process, a higher definition liquid crystal display device can be manufactured. However, since crystallization is performed by heating a substrate, the temperature of the substrate becomes extremely high (generally, 1000° C. or higher). On the other hand, a point of distortion of a glass substrate is about 600 to 700° C. Accordingly, glass cannot be used for a substrate in the HTPS process. In a thermal crystallization method using an element that promotes crystallization, crystallization of a semiconductor film is possible at a temperature that is about 500 to 600° C. lower than that of the above mentioned thermal crystallization method, as well: as a crystal with a continuous grain boundary can be obtained. However, compared to the other two crystallization methods mentioned above, a process is complex. Therefore, means of applying a necessary amount of heat for crystallizing an amorphous semiconductor film without damaging the glass substrate has been in need.
Taking into consideration the above circumstance, a laser crystallization method is used for crystallization of a semiconductor film over a glass substrate. In a laser crystallization method, since heating is performed locally by a laser for crystallizing the semiconductor film, a temperature of a process is relatively a low temperature (in general, 600° C. or lower). Therefore, a glass substrate can be used, and the process is not complex. By using the laser crystallization method, it has become possible to apply the necessary amount of heat for crystallizing the semiconductor film by laser light irradiation without damaging the glass substrate.
Here, the laser crystallization method is a method of crystallization in which an amount of heat is applied to an amorphous semiconductor film that is higher than or equal to a melting point of the amorphous semiconductor film by a laser, and heating a surface locally for a very short amount of time.
A polycrystalline semiconductor film formed by the laser crystallization method has inferior mobility compared to a monocrystalline semiconductor. This is considered to be due to crystal grain boundaries that are caused along with crystallization. Consequently, for a purpose of reducing crystal grain boundaries that exist in the semiconductor film to obtain crystallinity that is close to a monocrystal, development of techniques for controlling positions of the grain boundaries and enlargement of the crystal itself by controlling a location of nucleation in crystallization has been attempted. As one of such techniques, there is a method of forming a polycrystalline semiconductor film in which a semiconductor material is crystallized by SLG (Super Lateral Growth), by using a phase-shift mask with a stripe pattern and periodically modulating intensity of a linear pulsed laser light that is emitted to a surface of a sample (for example, Patent Document 1: Japanese Published Patent Application No. 2004-119919).
Note that the phase-shift mask is a mask of a quartz substrate with trenches, which is used in a phase-shift method. A phase-shift method is a method of improving resolution by shifting a phase of an adjacent pattern by 180°.